1. Field of the Invention
This invention relates generally to so-called aperture grilles, and more particularly relates to a grid apparatus for use with a color cathode ray tube.
2. Description of the Prior Art
In an ordinary color cathode ray tube, a grid apparatus is positioned adjacent to and opposed to the phosphor screen of the color cathode ray tube to determine a landing position of an electron beam on the screen. In a color cathode ray tube such as a Trinitron (Registered Trademark) tube, as shown in FIG. 1, a funnel portion 2 having a neck portion 1 is connected along its open end with a panel portion 3, thus composing an evacuated envelope 4. Within the envelope 4 near the panel portion 3 there is located a grid apparatus 7 called generally, for example, aperture grille or shadow mask opposed to a phosphor screen 6 formed on the inner surface of face plate 5 of the envelope 4. By this grid apparatus 7, each electron beam 9 corresponding to respective colors of, for example, red, green and blue (in the figure only one electron beam is illustrated) emitted from an electron gun 8 provided within the neck portion 1 impinges on the phosphor stripe corresponding to the desired color on the phosphor screen 6.
As, for example, shown in FIG. 2, this grid apparatus 7 comprises a frame 10 composed of a pair of spaced frame members 10a and 10b opposing each other, and left and right arm portions 10c and 10d supported therebetween. Across the front surfaces of the opposing frame members 10a and 10b of the frame 10, namely, the end surfaces of the frame 10, namely, the end surface thereof opposing the phosphor screen 6 (in FIG. 1), there is stretched a grid element 11. This grid element 11 is made of, for example, a thin metal plate, through which a number of slits are bored in parallel to one another with a predetermined interval in the same direction by selective etching technique. Both end edges of the thin metal plate in the extended direction of the slits are welded to the respective end surfaces of the front side of the frame members 10a and 10b in the frame 10. When the color cathode ray tube is placed in operation, the grid element 11 is apt to be expanded by heat generated due to, for example, the electron beam impinged thereon. In order to avoid slack and preserve a stretched state with a predetermined tension between the frame members 10a and 10b, the grid element 11 has to be stretched between the frame members 10a and 10b with a predetermined tension distribution beforehand. To this end, when the grid element 11 is stretched between both the frame members 10a and 10b or the thin metal plate with the aforesaid slits is attached therebetween, both of the frame members 10a and 10b are applied with loads from the outsides thereof, namely, from the sides opposing to the sides where both of the frame members 10a and 10b are facing each other so as to bend the frame members 10a and 10b inwardly, in other words, a so-called turn buckle or vise is applied thereto. Under this bent state, the grid element 11 is welded, and hence fixed, to both of the frame members 10a and 10b.
In order that the frame 10 may have mechanical strength enough to withstand the turn buckle upon the stretching operation and the loads due to the tension of the grid element 11 welded to the frame members and in order to obtain turn buckle and tension, with each having a predetermined tension distribution, the shapes and sizes of the respective frame members 10a and 10b and left and right arm portions 10c and 10d, and in addition, the connection positions of the left and right arm portions 10c and 10d to the frame members 10a and 10b are suitably selected.
The frame members 10a and 10b of the frame 10 are preferably each made of steel material such as a carbon steel and the like formed as, for example, L-shape in cross-section, while each of the left and right arm portions 10c and 10d of the frame 10 is preferably formed of a hollow metal tubular pipe to provide strength with a reduced weight. These left and right arm portions 10c and 10d are respectively comprised of both end portions 10.sub.c1, 10.sub.c2 and 10.sub.d1, 10.sub.d2, each being extended from the frame members 10.sub.a and 10.sub.b to the rearwards, and middle portions 10.sub.c3 and 10.sub.d3 curved horizontally to the outside from these end portions 10.sub.c1, 10.sub.c2, and 10.sub.d1, 10.sub.d2. The middle portions 10.sub.c3 and 10.sub.d3 are also curved as U-shapes within the planes intersecting the planes determined by both end portions 10.sub.c1, 10.sub.c2, and 10.sub.d1, 10.sub.d2, at predetermined angles. As a whole, the above left and right arm portions 10.sub.c and 10.sub.d are curved in three-dimensions and formed as complex shapes. These left and right arm portions 10.sub.c and 10.sub.d are welded at their ends to the rear end surfaces of the frame members 10a and 10b, where the connection positions thereof are selected to be Bessel points of the frame 10a and 10b or in the proximity thereof as determined in U.S. Pat. No. 3,638,063.
The grid apparatus 7 is attached to the inner peripheral surface of panel portion 3 of the cathode ray tube by way of stud pins not shown. Moreover, the grid apparatus 7 is provided with a temperture compensating means, not shown, which uses, for example, a bimetal mechanism to sense the temperature to permit, for example, the opening state of the left and right arm portions 10.sub.c and 10.sub.d, each being formed as the U-shape to be adjusted, whereby in response to the expansion and contraction of the grid element 11 due to heat, the grid element 11 is always stretched at a predetermined tension.
Regarding the conventional grid apparatus for use with the color cathode ray tube as mentioned above, in order that the predetermined mechanical strength may be obtained, the thicknesses of its frame members 10a and 10b and the diameters and the thicknesses of the hollow pipes of the left and right arm portions 10c and 10d and so on are selected suitably. This leads to a remarkable increase of its weight. Moreover, as the cathode ray tube is made larger, mechanical strength must also be increased, so this causes its weight to be increased much more. As the weight thereof is increased, the accompanying attaching means and the temperature compensating means accompanying therewith also become large, which causes its weight to be increased still further. Furthermore, the shapes or configurations of the left and right arm portions 10c and 10d are appreciably complex, the manufacturing process is made complicated, and the space shared by the whole of the grid apparatus is increased.